1. Field of the Invention
The present invention relates to semiconductor electrical fuse (e-fuse) technology, in general, and particularly to a novel method for programming e-fuses and enhancing their reliability in electronic circuits.
2. Description of the Prior Art
Properly programming (commonly called “blowing”) e-fuses require a carefully controlled level of programming current. Deviation of programming current from the optimal level results in “underblow” conditions where the fuse resistance is not as high as it should be or in “rupture” conditions where higher current physically disrupts the fuse structure leading to potential reliability problems during usage through the lifetime of the semiconductor chips.
FIG. 1(a) illustrates a circuit 10 including an e-fuse device 15 formed by conventional CMOS processing techniques for implementation in semiconductor integrated circuits, e.g., a data memory. As known in the e-fuse art, this circuit 10 includes a power source 12 (e.g., VFS) connected to programmable transistor device 25 (e.g., a FET) that is responsive to a control signal 20, e.g., VGS, to regulate the level of IDS current 30 flowing through the e-fuse 15 and the transistor 25 required to program (e.g., blow) the e-fuse thereby changing or customizing any connecting circuitry to meet the requirements of a specific application. For example, after the e-fuse has been blown, the circuit path of which it was a part no longer exists, and current is then directed along different pathways in the device.
As shown in FIG. 1(b), the e-fuse device 15 is a silicon based semiconductor structure having a cathode portion 18, a fuselink portion 17, and an anode portion 16. Typically, the fuselink is a narrow silicide structure, having dimension on the order of about 93 nm in width and a length of about 1.2 μm in length. The anode is silicided polysilicon having a width on the order of about 0.8 μm and a length on the order of about 2.0 μm, while the cathode is on the order of about 1.5 um wide and length is on the order of about 1.5 μm with the cathode connected to the programming transistor and the anode is connected to a high voltage power supply. At time of electrical fuse programming, i.e., applying a voltage to the anode and turning on the programming transistor to pass current through the fuselink, the silicide electromigrates from cathode to anode side and a structural change results (that is, the fuse is programmed). Suitable fuselink materials include, but are not limited to, cobalt silicide, titanium silicide, nickel silicide, palladium silicide, or other metal silicide material that displays electromigration characteristics. Other materials, with or without a silicide component, may also be suitable. A nitride or silicon dioxide material may form a top layer of the fuse structure shown in FIG. 1(b). The e-fuse structure may be formed in a variety of ways, for example, via a conventional semiconductor manufacturing processes which is compatible with advanced CMOS technology or other innovative methods that involve additional processing. The e-fuse circuitry may be designed in a variety of ways to improve the sensing of the status of the e-fuses.
For like circuits including e-fuses, deviation from the ideal programming current level of IDS is caused in two ways: 1) first by the process variations during the manufacture of the hardware; and 2) second, from the ambient conditions of the chip during the programming, notably temperature. An example of process variation induced distribution of programming current is shown in FIG. 2. FIG. 2 particularly is a graph 50 depicting the variation in IDS−VGS characteristics of an example semiconductor transistor device and particularly illustrates how the Ids current and Vgs voltage curve 75 varies at different chip operating temperatures, and particularly depicts the chip-by-chip variation of programming transistor I_on within one wafer.
As shown in FIG. 2, the operating temperature of the chip during programming could be affected by the ambient temperature and how much heat is being generated by the chip at the time of the programming, i.e., whether the chip is otherwise running at full speed inside a machine dissipating substantial heat or whether the chip is in a state close to standby. Given such a chip by chip response due to variations of the programmed transistor, FIG. 2 illustrates an I_prog range for the nominal e-fuse and an optimal Vgs variation.
In semiconductor chips where deviations from the ideal current range is sufficient, the e-fuses program in the underblow mode or rupture mode as described above. Since most or all fuses in such chips program outside the optimal programming window, typical circuit level redundancy solutions provide only a limited solution to the overall accuracy in the encoded data.
For example, if all fuses program in the rupture mode, all fuses are subject to the same reliability issue during the lifetime of the chips. Since the variation of the programming current affects all fuses in the same chip the same way, conventional redundancy solution of building a multiple number of fuses, each of which are identical among themselves, for the same bit still cannot avoid the inherent e-fuse programming reliability problem.
Similarly, if all fuses program in the underblow mode, improving the accuracy of the coded information will take building substantial number of redundancy e-fuses, each of which are identical among themselves, because each of the individual fuses have a fairly substantial chance of programming with low resistance. For this approach to be reliable, it would require a very large number of e-fuses and consequently, would require a large area of the chip for just one bit.
The problem of programming bits reliably becomes more acute when the information to be coded is critical to the chip's performance or to the performance of the equipment that the chip is controlling. Examples of such information are the information that is required to enable the rest of the chip or some features thereof; and the information that would determine the course of future actions and operations while a piece of equipment is in operation in the field.
Therefore, it would be highly desirable to provide such a method for programming an e-fuse device with an accuracy and reliability that are independent of the operating state of the chip.
It would also be highly desirable to provide such a method for programming an e-fuse device with an accuracy and reliability that are independent of inherent process variations caused during the manufacture of the chip.
It would be highly desirable to provide such a method for programming an e-fuse device with an accuracy and reliability while minimizing the area that the circuit uses by reducing the number of fuses and devices to a minimum number allowable.